Cardiac hypertrophy is a pathophysiological adaptation to a number of conditions, including hypertension. The ability of angiotensin-converting enzyme (ACE) inhibitors to inhibit cardiac hypertrophy has led to a number of studies demonstrating that angiotensin-II (AII) directly stimulates growth and division in cardiac cells. AII-stimulated cell division in neonatal rat cardiac fibroblasts has recently been shown to involve tyrosine phosphorylation of proteins in the signal transduction process. This proposal outlines experiments that will further investigate the process of AII-stimulated signal transduction by protein tyrosine phosphorylation. One of the proteins that exhibited an increased level of tyrosine phosphorylation in cardiac fibroblasts following AII stimulation was pl25FAK, or focal adhesion kinase (FAK), a protein believed to play a role in the formation of focal adhesions, which are the point of interaction between the cytoskeleton of the cell, and the extracellular matrix (ECM). Therefore, immunofluorescence will be used in cardiac fibroblasts to study changes in the localization of FAK and paxillin, a focal adhesion-associated protein, in response to AII stimulation, and also to follow changes in the structure of the cytoskeleton in response to AII stimulation. Since AII stimulates cell growth in cardiomyocytes, as opposed to cell division in cardiac fibroblasts, differences in the levels of FAK tyrosine phosphorylation following AII stimulation in these two cell types will be determined. If FAK is tyrosine phosphorylated following AII stimulation in cardiomyocytes, immunofluorescence will be used to study FAK localization and cytoskeletal structure in these cells as well. Since FAK is involved in the structure of focal adhesions, which are the points of contact between the internal cytoskeleton and the ECM, the role of the ECM in modulating the protein tyrosine phosphorylation process initiated by AII binding will also be studied by growing neonatal rat cardiac fibroblasts and cardiomyocytes on a number of different ECM attachment substrates. Differences in FAK tyrosine phosphorylation and localization and structural changes in cytoskeleton will be examined in these cells. The results of the experiments outlined in this proposal will serve as the basis for further work into whether FAK is necessary for AII-induced growth in cardiac cells.